Analysis of TiN Precipitation Behavior during Solidification of Bearing Steel

The inclusions in bearing steel continuous casting bloom were observed,and the large size TiN inclusions were found in the bloom.The effects of Ti and N elements segregation at the solidification front on precipitation of TiN inclusion was studied on the basis of thermodynamics and dynamics.The thermodynamics analysis results show that there is no TiN precipitation above liquidus.When the solid fraction is above 0.65~0.75,the TiN could be precipitated at the solidification front because of Ti and N segregation.With the decrease of Ti and N content in steel,the formation of TiN will be delayed and the size and number of TiN will be decreased.The dynamics analysis results show that with the increase of cooling rate,the size of TiN inclusion during solidification is significantly reduced.When the cooling rate is higher than 50 K/s,the calculated size of TiN is 2.4 μm,and this value will be more than 7.6 μm when the cooling rate is less than 5 K/s.The size of TiN is only tens to hundreds of nanometers in the case of TiN precipitating in solid steel.The TiN inclusion with more than equivalent radius of 2.5 μm was not observed in the surface layer of bloom,but the equivalent radius of 6.77 μm and 8.46 μm were found in the 1/4 thickness and the center of bloom,respectively.These results indicate that the large size TiN inclusions precipitate during solidification.